Our objective is a better understanding of both the conditions that lead to insertional mutations and the molecular basis of those mutations. The specific aims are: 1) To understand the mechanism of action of genetic modifiers of white-apricot (wa), a mutation caused by the insertion of thee transposable element copia, at the molecular level. New modifiers of wa will be isolated, and the effects of modifiers of wa on the expression of wa and copia itself will be assessed by examination of the RNA and protein products of gene fusions. Specific promoter or polyadenylation sequence elements through which these genes might affect copia expression will be identified by examining the effects of in vitro mutations. 2) To identify the genetic and environmental factors which influence the transposition of copia. The transposition of a marked copia element will be assayed by a novel test; these experiments are designed to be the first to assign a rate to copia transposition. Genetic suppression in Drosophila is fundamentally different from suppression in prokaryotes, which usually involves the suppression of point mutations within coding regions by second-site mutations in tRNAs. In contrast, suppressible alleles in Drosophila are generally of spontaneous origin and are associated with the insertion of transposable elements into noncoding DNA. Available information indicates that suppression in Drosophila involves the alteration of factors which act in either transcription or RNA processing, and that the study of suppression in Drosophila will identify the products of suppressor loci as trans-acting factors which act at these stages of gene expression. The transposable element copia is a member of the broad class of structurally homologous genetic elements that includes thee transposable Ty elements of yeast, vertebrate retrovirus proviruses, and a number of distinct Drosophila transposable elements. Many spontaneous mutations in Drosophila are caused by the insertion of transposable elements, and many of these are suppressible. The apricot allele of the white locus (which affects eye pigmentation) has been chosen for this study of suppression in Drosophila. wa is associated with the presence of copia within an intron of white. A major advantage of wa is that small deviations from its intermediate level of pigmentation are readily discerned by inspection. The mutant phenotype of wa is enhanced (pigmentation is further reduced) by En(wa) and su(f)), but suppressed by su(wa).